1. Field of the Invention
This invention generally relates to light fixture components for lighting fixtures. In specific embodiments, the invention relates to a reflector for use with an overhead light source that includes a plurality of undulations or curves in the vertical dimension on at least a portion of its inner and outer surface. These undulations serve to diffuse light that emanates from the light source. The outer surface of the reflector also includes a plurality of prisms for internal prismatic reflection.
2. Description of the Related Art
There are various reflectors available for use with overhead lighting fixtures, particularly for commercial, industrial, institutional and residential lighting purposes. It is often desirable for these reflectors to reflect light from a light source located within the reflector to produce even illumination of a plane. The term “reflector” has traditionally been used to refer to metal reflectors, which are reflectors in the true sense of the term—in that they reflect light incident to their exposed surface, are opaque, and are not capable of transmitting light. For example, some conventional reflectors provide the desired light distribution by featuring opaque reflective surfaces that do not transmit rays.
In recent years, however, the term “reflector” has also been used to refer to transparent devices that incorporate structures such as prisms, so that the devices reflect as well as refract light. Transparent devices without the modified surface structures would only refract light, and would not be useful as reflectors. The term “reflector” or “light fixture component” is used in this patent to refer to this second type of reflector and the phenomenon of the reflecting that occurs, referred to as “total internal reflection.” The principals of refraction and total internal reflection combine to mimic the behavior of an opaque reflector. For example, some transparent reflectors provide prismatic reflection through the use of 90-degree prisms or external prismatic surfaces that are a combination of 90-degree and curved prisms. The reflection only occurs for light entering from within a small zone. This is illustrated by the schematic at FIG. 11. As those of ordinary skill in the art will recognize, if a light source is larger than a particular size, some light will pass through the reflector because light will strike the inner surface of the reflector at an angle that does not result in total internal reflection at both exterior prism faces. In other words, outside that zone, the light will be refracted and transmitted rather than undergo total internal reflection; however, the transmitted light may be useful as uplight.
One challenge faced by designers of reflectors is that it is difficult to create a design that works well with many different sizes and types of lamps and lamp positions. Such a versatile design is typically preferred from the manufacturer's standpoint because there is less tooling involved and fewer inventory control issues. This in turn may allow the manufacturer to offer the reflector at a reduced price, providing cost savings to the end user.
The shape and size of a particular reflector is often driven by the shape and size of the light source with which it is to be used. For example, luminaire housings employing linear sources such as fluorescent lamps tend to be linear or square. Point sources are often used in connection with reflectors that are surface of revolution or bell-shaped.
It has also been found that the use of 90-degree prisms in connection with transparent reflectors is particularly efficient for situations such as industrial lighting applications. Ninety-degree prisms typically allow only a small percentage of light to pass through the reflector (although some light naturally passes through the reflector, primarily as a result of originating too far off axis as described above).
Ninety degree prisms disposed on the outside surface of reflectors have been used for several decades. See e.g., U.S. Pat. Nos. 365,974, 563,836, and 4,839,781, which are all hereby incorporated by this reference. The use of such prisms is an effective optical control technique. Prisms have been disposed vertically on outer reflector surfaces, as well as horizontally. Additionally, in order to enhance the optical control, the interior surfaces of reflectors may be smooth, vertically fluted, textured, or stepped with interior contours to help direct light to the prism faces.
Prisms may be provided in various materials, such as glass, plastic, or acrylic. An acrylic prism approach is advantageous primarily because of its high efficiency. The acrylic absorbs very little light as it passes through. When light enters from within the reflection zone, it is reflected with significantly higher efficiency than a typical aluminum anodized reflector. The acrylic design naturally creates an uplight component that is often desirable as well. Uplight reflects from the ceiling, thereby reducing the contrast between the bright light source and its background. This reduces the potential for glare, softens shadows, and generally makes for a better lighting condition. Another advantage of an acrylic reflector is that it glows all over. This effectively increases the size of the light source from a glare perspective.
Another factor that designers of reflectors must consider is that the size of the light source dictates the size of the zone into which light is reflected. In many cases, the use of a large light source creates a “hot spot.” The light from the source is reflected by the reflector due to total internal prismatic reflection and directed predominantly toward a single narrow zone below the light source, i.e., the zone encompassing “nadir.” (Similarly, if the device were inverted, the same phenomenon could force the light to be directed predominantly toward a single, narrow zone above the light source, i.e., the zone encompassing “zenith.”) In both cases, this phenomenon creates an undesired “hot spot” directly below or above the light fixture. Even a small amount of light can result in a significant candela spike at these locations due to axial symmetry.
The uppermost portions of the reflector tends to contribute most to the hot spot due to that portion's proximity to the lamp and also because the uppermost portion is curved or “aimed” inward. The result is that light that is internally reflected from the upper portion of the reflector is projected toward nadir.
Existing bell-shaped reflectors have a tendency to reflect or redirect light toward the axis of revolution, resulting in a disproportionately large contribution of light at nadir relative to directions outward and away from the axis of revolution. This causes a spike in the intensity distribution of the reflector, a “hot spot,” which prevents even illumination. A reflector that creates a “hot spot” will present a light puddle, or an undesirable bright area of illumination directly beneath the luminaire when compared with the entire surface that is being illuminated.
There have been numerous attempts to avoid the problem of hot spots, although some have been more effective than others. For example, efforts have been made to texture the inner surface of reflectors (for example, by sand blasting, acid etching, or peening), but these efforts often result in greater manufacturing expense. They may also result in a general diffusion that causes a greater percentage of light to transmit through the reflector body while reducing the downlight efficiency of the luminaire.
Additional efforts include providing “stepped” interior contours to alter the direction of the reflected light in the vertical dimension only, however this method requires more plastic than other methods. Reflectors having such a “stepped” inner surface were analyzed and also found to change the direction of light, thereby increasing sensitivity with respect to lamp position. Designs that primarily diffuse light by sending it into a broad vertical zone, rather than additionally altering the direction are preferable because they can accommodate a broader range of lamp types and positions. Additionally, the stepped inner surface of the prior art reflectors includes steps only on the uppermost, inside portion of the reflector creating a discontinuity of appearance in the vertical direction. These steps are not provided over the entire interior surface of the reflector and are not present on the outer surface, thereby increasing the amount of plastic required to maintain a minimum wall thickness.
Accordingly, there remains a need in the art for a reflector that alleviates the above-described hot spots, while maximizing the amount of reflected light and minimizing the amount of plastic required. The improvements offered by the present inventors help alleviate the problems described in ways not addressed by the prior art.